An Improved Method for Using MgII to Estimate Black Hole Masses in Active Galactic Nuclei

TL;DR

This paper introduces an improved method for estimating black hole masses in active galactic nuclei using the MgII emission line, correcting for biases and validating its accuracy across a broad redshift range.

Contribution

The authors develop a correction technique for MgII-based black hole mass estimates, enhancing their reliability and extending their applicability to higher redshifts.

Findings

MgII-based masses can be corrected for Eddington ratio dependence.

The intrinsic Eddington ratio distribution remains narrow (~0.3-0.4 dex).

The method increases the dispersion slightly but maintains overall accuracy.

Abstract

We present a method for obtaining accurate black hole (BH) mass estimates from the MgII emission line in active galactic nuclei (AGNs). Employing the large database of AGN measurements from the Sloan Digital Sky Survey (SDSS) presented by Shen et al., we find that AGNs in the redshift range 0.3-0.9, for which a given object can have both H-beta and MgII line widths measured, display a modest but correctable discrepancy in MgII-based masses that correlates with the Eddington ratio. We use the SDSS database to estimate the probability distribution of the true (i.e., H-beta-based) mass given a measured MgII line width. These probability distributions are then applied to the SDSS measurements from Shen et al. across the entire MgII-accessible redshift range (0.3-2.2). We find that accounting for this residual correlation generally increases the dispersion of Eddington ratios by a small factor (~0.09 dex for the redshift and luminosity bins we consider). We continue to find that the intrinsic distribution of Eddington ratios for luminous AGNs is extremely narrow, 0.3-0.4 dex, as demonstrated by Kollmeier et al. Using the method we describe, MgII emission lines can be used with confidence to obtain BH mass estimates.